The present invention pertains to petroleum processing and, specifically, to the art of disposal of effluents resulting therefrom.
Crude oils are exceedingly complex mixtures, consisting predominantly of hydrocarbons containing sulfur, nitrogen, oxygen, and metals as minor constituents. While it is desirable to recover the hydrocarbon constituents in their pure form, realistically it is very difficult to isolate pure products because most of the minor constituents occur in combination with carbon and hydrogen. Separation of impurities such as those listed above generally requires expenditures of valuable resources such as time, chemicals, energy, and money. Therefore, it is the constant goal of the petroleum processing industry to optimize impurity removal procedures, equipment and resources in order to eliminate those impurities which have the most degrading effect on the end products.
Perhaps the most ubiquitous impurity encountered in petroleum processing is sulfur. The presence of sulfur in petroleum products and, indeed, in the crude feedstock itself generally increases the corrosive characteristics thereof, and forms harmful and noxious reaction products upon combustion. In particular, the presence of sulfur-containing compounds reduces the combustion characteristics of gasoline and may render fuel oil unusuable in many places due to local regulations on the amount of sulfur allowed therein. Consequently, at nearly every stage of production measures are taken to either reduce the amount of sulfur or to render the sulfur-containing compounds inoffensive.
One method for removing sulfur-containing compounds--hydrogen treating of petroleum fractions--has been known since the 1930's. However, it was not until the advent of catalytic reforming, which made inexpensive hydrogenrich off gas available, that hydrogen desulfurization developed to commercial level. Presently, hydrogen desulfurization is primarily associated with a catalytic reaction using cobalt molybdate on an alumina carrier. The feedstock is mixed with recycle and make-up hydrogen and heated to 400.degree.-850.degree. F., then charged to a fixed bed reactor at 50-1500 psig.
Hydrogen treating is now used extensively to prepare reformer feedstock and to some extent for catalytic cracking feedstock preparations. It may also be used to upgrade middle distillates, cracked fractions, lube oils, gasolines, and waxes. Hydrodesulfurization, however, is a high energy-consuming process which also requires a supply of hydrogen.
Moreover, a major effluent resulting from hydrodesulfurization is hydrogen sulfide, H.sub.2 S,--a flammable poisonous gas. Even though hydrogen sulfide may simply be burned-off into the atmosphere, legislation in recent years has effectively limited this method of disposal because of the formation of sulfur dioxide which is intensely irritating to the eyes and respiratory system. Accordingly, alternate means for disposing of hydrogen sulfide have been developed and implemented.
The primary method of disposing of hydrogen sulfide is to convert the sulfur-bearing gas to elemental sulfur and water by, for instance, the Claus process. While this alternative may appear somewhat attractive since elemental sulfur is a saleable commodity, the Claus process requires construction of sulfur plants, expensive catalyst, and energy. Furthermore, the market for elemental sulfur is not so extensive as to be able to absorb all the elemental sulfur currently produced without depressing the price therefor. Finally, the Claus process itself is fraught with some difficulty in that approximately 3% of the reaction product is, again, the noxious sulfur dioxide which must be further treated by, for instance, a tail gas treating process, in order to reduce the level of SO.sub.2 effluent to within the Environmental Protection Agency Standard of not more than 250 parts per million on a dry oxygen-free basis. Consequently, while the Claus process is still a viable alternative for disposing of hydrogen sulfide gas, it has become less attractive because of the cost of carrying out the process and because of the decrease in demand for the elemental sulfur.
Another method for treating petroleum to reduce the degrading effects of sulfur is chemical processing to "sweeten" sulfur compounds contained in the particular fractions, e.g., the mercaptans which are designated by the formula RSH. "Sweetening" denotes that mercaptan sulfur compounds are removed from a refinery stream, or else the mercaptans are converted to less objectionable disulfide compounds, e.g., R-S-S-R, R-S-S-R', etc. A particularly important process employed today is the sweetening of kerosine by the MEROX process to obtain jet fuel.
Whether sweetening is undertaken by solvent refining processes or by fixed bed adsorption, a caustic solution is generally first used to convert the mercaptan to the ionic state, RS.sup.-, e.g., as in the cresylate process. Caustic solution is also helpful in that it removes naphthenic acids and other organic acids in general, and other sulfur compounds from cracked petroleum products and petroleum distillate. In fact, caustic treating of petroleum products has been used to improve odor and color nearly as long as the industry itself has been in existence. Numerous equipment modifications and processes have been designed to implement caustic treatment of process streams. Unfortunately, since caustic is quite harmful to organic tissue extreme care must be taken in the use and disposal of alkaline solutions such as sodium hydroxide solutions in order to protect the waterways, rivers, subterranean water formations, and, in many places, the oceans and surrounding seas of industrial areas from caustic pollution. This presents a significant problem to the industry because of the great volume of caustic used in petroleum processing and because all of the solutions used must eventually be discarded as a nonregenerative caustic or as the spent liquor resulting from a regenerative process.
To date the industry generally uses two methods to dispose of spent caustic--neutralization and incineration, incineration being a relatively new trend in waste alkaline liquor disposal. Incineration disposal presents certain advantages over neutralization disposal in that it is, first of all, environmentally cleaner since acid neutralization has a residual OOD (Organic Oxygen Demand) for the napthenic acids; while other advantages include such benefits as lower capital investment and less operating space required (i.e. ground area). Furthermore, incineration disposal is easier to operate. On the negative side, cost of operation of the incinerator units is high because of the energy required to maintain the elevated temperatures necessary to maintain combustion of the predominantly aqueous solution of alkaline waste. Commercial units presently in operation make use of the combustion of fuel oil, and natural gas to sustain the evaporation of the aqueous parts of the waste liquor and then furnish mostly carbon dioxide, CO.sub.2, to form the innocuous carbonates of sodium or other alkali metals for disposal. Refined fuel oil and natural gas are very expensive means for disposing of the volumes of caustic discarded as a result of petroleum processing and they are both very valuable commodities in terms of consumer-useable energy sources.
In a co-pending application Ser. No. 238,309, filed Feb. 26, 1981 by the same inventors, a method is disclosed whereby the great volume of alkaline waste liquor which results from petroleum processing, can be safely disposed of by expending reduced amounts of commercially-useable and expensive fuel supplies; and whereby innocuous sulfates are produced from potentially harmful and/or noxious sulfur-containing fuels. The disclosure of the above-referenced co-pending application does not, however, solve the problem of production of excess sulfur containing effluents, nor the problem of insufficient sulfur in the particular fuel to neutralize all the caustic produced at a particular processing plant. It is, therefore, the object of the present invention to resolve both of these problems utilizing processing effluents which are otherwise unuseable or undesirable.